Firefighters were called to a report of an explosion on the sixth floor of the hospital around 2:15 a.m., Durham Fire Department spokeswoman Sierra Jackson said. The firefighters discovered there had been no explosion and the fire had been extinguished by the hospital sprinkler system.

The cause of the fire was still under investigation, Jackson said later Tuesday morning.

Hospital officials were still investigating exactly where the fire occurred and how, said Katie Galbraith, hospital chief of operations.

The hospital was operating normally several hours later.

The sixth floor of Durham Regional is a unit operated by Select Specialty Hospitals, a company that provides care for critical and complex cases that require more attention than conventional patients, the Raleigh News and Observer newspaper reported. Other patients in the 30-bed unit were moved to other parts of the hospital, which is owned by Duke University. The company did not immediately return calls for comment.

Some other patients were moved because of flooding caused by the sprinklers.